    Patent Document 1: JP 2007-5279 A    Patent Document 2: JP 2000-164252 A    Patent Document 3: JP 2004-281316 A    Patent Document 4: JP H06-9141 B    Patent Document 5: JP 2001-210360 A    Patent Document 6: JP 2001-351615 A    Non-Patent Document 1: J. Power Sources, 81-82, (1999), 853    Non-Patent Document 2: J. Electrochem. Soc., 124, (1977), 1240-1242
In recent years, the development of electronics techniques is remarkable, and portable electronic appliances are being reduced in size, weight, and thickness, and are provided with multiple functions. With these trends, batteries, which are power supplies of electronic appliances, are strongly demanded to reduce in size and weight and to improve reliability. In order to meet these demands, a multilayer lithium ion rechargeable battery was proposed, in which multiple positive electrode layers and negative electrode layers are laminated thorough an electrolyte layer. Because the multilayer lithium ion rechargeable battery is assembled by laminating a battery cell having a thickness of a few μm, reductions in size, weight, and thickness of the battery can be readily realized. The all solid state lithium ion rechargeable battery using a solid electrolyte instead of a liquid electrolyte is free from the leakage and exhaustion of the electrolyte, and highly reliable. Moreover, because the battery uses lithium, the battery can obtain high voltage and high energy density.
An all solid state lithium battery using an inorganic solid electrolyte for the electrolyte layer is prepared by in turn laminating the positive active material layer and the negative active material layer through the electrolyte layer to form a multilayered product. In this all solid state lithium battery, various problems have been reported in the past.
For example, in Non-Patent Document 1, it is reported that when LiCoO2, which is a positive active material, and LiTi2(PO4)3, which is a solid electrolyte, are laminated and baked, both react with each other in the baking process to produce an impurity layer formed of a compound such as CoTiO3, Co2TiO4, or LiCoPO4, which is not contribute to charging and discharging reactions, at the interface of the positive electrode layer and the electrolyte layer. This impurity layer is formed to cause a problem that energy loss is large, or the product does not function as a battery, because the interface resistance is high.
In Patent Document 1, a technique is disclosed in which specific materials are used for an active material and a solid electrolyte, whereby the above-described formation of the impurity layer is suppressed, an active material/solid electrolyte interface is formed, which is electrochemically active, and an all solid lithium ion rechargeable battery having a small internal resistance and high capacitance is provided. Preferably, the following materials are used as materials for the active material and the solid electrolyte:
positive active material: LiMPO4 
(where M is at least one kind selected from a group formed of Mn, Fe, Co, and Ni)
solid electrolyte: Li1+xMIIIxTiIV2−x(PO4)3 
(where MIII is at least one kind selected from a group formed of Al, Y, Ga, In, and La, 0≦x≦0.6)
negative active material: FePO4, Li3Fe2(PO4)3, and LiFeP2O7.
Patent Document 1 describes that when a multilayered product having the positive active material layer, the electrolyte layer, and the negative active material layer formed of these materials laminated is analyzed by X-ray diffractometry, any components other than the components forming the active material layer and the electrolyte layer are not detected.
FIG. 12 is a cross section depicting a lithium ion rechargeable battery before described in Patent Document 1, showing an interface of a positive electrode layer and an electrolyte layer. The battery shown in FIG. 12 is produced according to a process described below. A positive active material powder formed of LiCoPO4 and a solid electrolyte powder formed of Li1.3Al0.3Ti1.7(PO4)3 are prepared, they are separately mixed with a binder, a solvent, and a plasticizer to form slurries to prepare green sheets. After the prepared positive active material green sheet and the solid electrolyte green sheet are laminated, the sheets are baked at a temperature of 900° C. to form a multilayered product formed of an electrolyte layer 103 and a positive electrode layer 102, and gold is sputtered onto the surface of the positive electrode layer 102 to form a metal layer 101.
In the lithium ion rechargeable battery produced according to Patent Document 1, no impurity layer interfering with the charging and discharging reactions of the battery is formed at the interface of the positive electrode layer 102 and the electrolyte layer 103. In addition, also at an interface of a negative electrode layer and an electrolyte layer, not shown, no impurity layer is formed. Moreover, including the impurity layers interfering with charging and discharging reactions, an area formed of a reaction product is not formed at the interface of the electrode layer and the electrolyte layer. On this account, the interface bonding between the electrode layer and the electrolyte layer is not strong, and it is incapable of providing a sufficiently large contact area at the interface of the electrode layer and the electrolyte layer. Therefore, there is a problem that excellent charging and discharging cycle characteristics cannot be obtained because of the occurrence of delamination caused by long time use of the battery to increase internal resistance.